The present invention relates to a sprayable coating composition applied to fiber reinforced plastic (FRP) to enhance the surface of molded FRP parts and better prepare those parts for further coating or painting applications.
It is known that various fiber reinforced plastic (FRP) parts such as cured sheet molded compounds (SMC) can form cracks and sink marks which affect surface appearance. SMC is produced from unsaturated polyester or vinyl ester resin primarily. It is formulated with calcium carbonate filler, fiberglass for reinforcement, one or two catalyst, and usually an internal release agent like zinc stearate. The material is "B-staged" (thickened to a semi-solid) in long sheets which are rolled up for storage. When ready to mold, it is cut to mold size, placed into the mold which experiences clamp pressure and heat which fully cures the product in its final shape. Cycle times are usually 3-5 minutes depending on part size. After molding the "Stamped" parts usually have flashing that needs to be removed (usually by sanding), and/or may experience some type of processing like routing-out access holes, etc. This leaves raw edges or spots on the molded piece. When conventional paints are used, solvent can become trapped in these areas and subsequently "pop-out" when heated during normal baking processes. This creates a great deal of unusable parts. This phenomenon is greatly enhanced/accelerated by use of glass bubbles in the SMC formula to provide "low density" parts in an effort to reduce vehicle weight.
Accordingly, it has been suggested that a thin coating made from a liquid rubber and liquid epoxy polymer be used as a primerlike coating to inhibit propagation of micro cracks, reduce porosity and reduce sink marks in FRP parts and to provide a suitable smooth surface for automotive body panel applications that serves as a substrate for further paint applications. See, McGarry U.S. Pat. No. 5,358,779, issued Oct. 25, 1994.
It is also known through the assignee of the present application that a sprayable, high solids, low-volatile filler composition may be used as a coating on a variety of substrates including metals such as iron and aluminum, concrete, wood, fiberglass and polyester fillers, etc. See, for example, Parish et al U.S. Pat. No. 5,371,117, issued Dec. 6, 1994 which discloses a sprayable filler composition having a first filler/glazing component formed from a mixture of a resin, a filler, a mixture of thixotropic clays, activated fumed silica, hollow glass microspheres, and a ketone solvent and having a second catalyst component.
While Parish et al discloses that the sprayable, high solids, low-volatile filler composition may be used on a variety of substrates, no mention is made of molded fiber reinforced plastic (FRP) parts. Nor would one necessarily expect a high solids, low-volatile filler composition to adhere to a molded fiber reinforced plastic (FRP) part. When such a part comes out of the mold it is hard to find a coating which will adequately adhere to it. Most coatings have difficulty adhering to an unprepared SMC surface due to the internal mold release commonly used. Likewise, as pointed out in McGarry, it is difficult to find a coating which will not only adhere but also reduce propagation of micro cracks, reduce porosity, reduce sink marks and otherwise enhance surface appearance of molded FRP parts. McGarry teaches that a rubbery coating is necessary for that purpose. But, rubbery coatings have other problems in terms of cure time, hardness, and durability.
Accordingly, the need remains for an improved coating for molded FRP parts to enhance the surface of the molded FRP part and better prepare the part for further coating or painting application.